1. Field of the Invention
This invention relates to manufacturing processes, their control, and to measuring devices for use in the control of manufacture. More especially, it relates to monitoring polymerization processes, and to a control device embodying a rheometer or rheometers and the use of the device in controlling polymerization to obtain a polymer with desired properties.
2. Background Art
Although the invention will be described in detail with special reference to the manufacture of a synthetic polymer, it has utility in any application in which the rheological properties of a material in viscoelastic fluid form, for example in a melt, are related to the suitability of the material for a particular end use.
If a polymer, for example polyethylene, is to be suitable for certain applications, for example packaging film, it must satisfy certain requirements in processing and in final product quality. Film, for example, should be glossy and optically clear, i.e., free from haze, so that the contents of a package may be inspected and the package presents an attractive appearance to the customer. In addition to achieving these properties, it is also important to improve manufacturing rates without adversely affecting the quality of the product. Polymer melt properties may limit the rate of manufacture of commercially desirable products. The melt characteristics of the polymer affect the maximum production rates consistent with good quality product in, for example, melt-blowing film and extrusion coating processes.
It is known that the molecular weight distribution (MWD) and the degree of long chain branching (LCB) of a polyethylene are properties that affect the properties of the film made from such materials, and that there is a correlation between melt rheology and these properties--see Shida, et al., Polymer Engineering and Science, 17, 769, 1977. If clear film is desired, LCB is avoided, and/or MWD kept narrow, so far as possible. Conversely, when a high melt strength is required, as in extrusion coating, or the manufacture of thick or wide film, LCB and a wider MWD are desirable. In practice, polymer manufacturers have, however, employed melt index measurements or resorted to evaluating the optical properties of polymers such as polyethylene by blowing a film from a sample of the resin and measuring the haze.
The latter procedure has many disadvantages. Apart from its being an indirect measurement, with haze also possibly resulting from incorrect processing technique, as well as from the inherent properties of the polymer, measurement by film blowing and product inspection inevitably introduces a considerable time lag between the manufacture of polymer and its testing. The latter disadvantage also applies to melt index testing. Accordingly, considerable substandard quantities of polymer may have been produced, resulting in waste, before this fact is appreciated.
Patent literature contains certain disclosures of means for examining fluid material elasticity by use of pressure measurements conducted in narrow die systems. U.S. Pat. No. 4,241,602 addresses a continuous flow rheometer utilizing a slit or capillary tube for the measurement of pressure changes in a fluid with respect to viscosity and elasticity. The method taught utilizes pressure measurements within the capillary or slit for extrapolation of an exit pressure from which is calculated "first normal stress difference" which is said to be a measure of elasticity. U.S. Pat. No. 4,624,132 addresses rheometer design and a method of determining extensional viscosity and elasticity of molten polymer or other fluid by use of a combination of converging flow and diverging flow pressure measurements through capillary or slit dies having a particular configuration. The method is taught for use in design of machinery for the performance of polymer melt flow processing operations. Both designs presented contain complex configurations and require close attention to operational details thus permitting uncertainty of result and an unacceptable degree of reliability for use in manufacturing processes.
There accordingly remains a need for a more reliable and rapid method of measuring the rheological properties of a material to yield desired information about its structure.
There also remains a need for a simpler and more cost-effective method of obtaining information about a material's structure and integrating this information into the manufacturing process.